Sexual and parasexual genetic analysis has permitted the detection, characterization and genetic localization of a number of mammalian cellular genes which participate in endogenous type C virus expression. Seven classes of genes have been identified: (1) endogenous cellular DNA sequences homologous to type C viral RNA; (2) integration sites of retroviruses in mammalian chromosomes; (3) growth factor receptors; (4) restriction genes which delimit viral replication; (5) enzyme structural genes; (6) cell surface antigens including major histocompatibility complex (MHC); (7) cellular transforming genes (onc) resecured by defective retroviruses. Examples of each of these gene classes have been detected and mapped in human and feline model systems. The allozyme genetic signature as a definitive characteristic of cultured cells has been applied to monitors of cell contamination, quality control of in vitro transformation and identification of tissue origin in human embryonic development. Population genetic analysis of human feline, and murine populations has revealed evolutionary conservation of linkage associations and the characters of genetic variance.